1. Field of the Invention
This invention relates to a sputtering target for use in fabricating an integrated circuit (IC) device, and more particularly, to a sputtering target for forming a thin film of an alloy.
2. Description of Related Art
Thin films of alloys deposited by the sputtering technique are widely used as electrode or wiring materials, diffusion barriers and thin-film resistors of semiconductor IC devices and other IC devices.
For example, as materials for electrodes of MOSICs there are used in many cases polycides each comprising polycrystalline silicon and thereon a silicide of a high melting point metal such as tungsten silicide (WSi), molybdenum silicide (MoSi), and titanium silicide (TiSi). Thin films of alloys such as WSi, MoSi and TiSi are formed by the sputtering process including depositing an alloy material on a substrate by using it as a target.
Likewise, diffusion barriers are often formed in the form of a thin film of an alloy such as titaniumtungsten (Ti-W) by sputtering a target of the alloy. For a specified use, a thin film resistor of siliconchromium (CrSi) also may be formed by sputtering a target of the alloy.
The term "sputtering process" used herein means a process wherein: An inert gas such as argon (Ar) is supplied at a background pressure of a few millitorr into a chamber maintained at a high vacuum of usually about 1.times.10.sup.-7 torr, and subjected to DC-discharge. The produced argon ions (Ar+) strike a target mounted on the cathode, whereby target atoms are sputtered therefrom, and eventually deposited as a thin metal film onto an oppositely-disposed substrate.
An alloy target suitable for use in the above-mentioned process can be prepared generally by uniformly mixing powdered components of the alloy of a high purity and sintering the mixture under specified conditions of temperature, pressure and atmosphere, or melting it by using an electron beam. The thus-obtained target is homogeneous and its composition is uniform throughout.
In the above-mentioned targets in the prior art, however, there is encountered a problem that during the course of forming a film by sputtering with such a target as mentioned above, the composition of the thin film formed on the substrate varies as the target wears away, resulting in poor characteristics and quality of products.
The above-mentioned phenomenon that a film is being formed with its composition varying can be explained by the following exemplification: With a two-component alloy target whose components are elements greatly different in mass number, such as WSi, MoSi, TiSi, TiW or CrSi (Ratio of the mass numbers is about W/Si 6.5, Mo/Si = 3.4, Ti/Si = 1.7, W/Ti = 3.8, and Cr/Si 1.9, respectively), one component of smaller mass number, compared with the other larger mass number, tends to undergo scattering resulting from collisions with argon ions on the way from to the target to the substrate, and accordingly the thin film formed on the substrate becomes different in composition from the target, and thus the amount of the component of smaller mass number in the formed film lowers.
As the target wears away, erosion becomes deeper, and the distance of sputtered particle travel to the substrate becomes longer. On the other hand, the mean free path of Ar+ is the same if pressure and temperature remained unchanged. It follows that the prolonged distance contributes to increasing the number of collisions, and the more easily the smaller mass number component undergoes scattering. With a homogeneous target, therefore, the composition of a film formed on a substrate varies so that the amount of the smaller atomic mass number component decreases as the target wears away. It has been found from studies by the inventor that with Ti-W as a target in which 10 wt % Ti is uniformly contained, the weight percent of the smaller mass number component (Ti) in a thin film formed on a substrate using the sputtering target in the last stage of erosion was lowered by 30 to 40% by weight compared with the amount of Ti in another thin film formed on another substrate using the same sputtering target in the initial stage of erosion. FIG. 6 shows an example of the data obtained, in which the datum indicated by A is of the initial target stage and the datum indicated by B is near the last target stage after erosion.
As described above, with a homogeneous target in the prior art, the composition of an actually-formed film on a substrate varies considerably with the progress of erosion of the target. In particular, the greater the ratio between mass numbers of components of the target is, this tendency appears more remarkably, reflecting various defects: For example, electrodes made from WSi, MoSi or TiSi may suffer peeling and swelling after thermal treatment. Diffusion barriers from TiW may result in poor heat resistance. As for thin film resistors made from CrSi, problems with stability of the resistance value may arise. This leads to impairment in yield and reliability of products.
The phenomenon is considered to be the same in a thin film of a three or more component alloy deposited by using a three or more component alloy target.
When a superconductive thin film of an alloy of La (lanthanum) - Ba (barium) Cu (copper) - O (oxygen), of La - Sr (strontium) Cu - O, or of Y (yttrium) - Ba - Cu - O, etc. is formed on a substrate by sputtering using the same composition alloy target, the weight percent of the component O (oxygen) to the total weight, of which the O (oxygen) mass number is the smallest of the components, is varied between the initial stage to the last stage of the target, thereby unfavorably changing the critical temperature of the superconductive thin film.